1. Field of the Invention
The present invention relates, in general, to the field of electronics circuits for surveying instruments. More particularly, the present invention relates to a self compensation circuit for variations in reference levels for an electronic compass module in a laser based surveying instrument.
2. Description of the Related Art
A conventional laser based surveying apparatus developed and currently marketed by Laser Technologies, Inc. of Englewood, Colo. is the Criterion.TM. series of surveying instruments. The Criterion.TM. instrument includes, among other things, a laser transmitter and receiver, a microprocessor, a numeric keypad for data entry and instrument control commands, an internal fluxgate compass for bearing/azimuth measurement, a tilt angle sensor for inclination measurements, an aiming or spotting telescope mounted on the housing, a data collector, and serial port for data transfer. This instrument measures and computes heights, horizontal distances, vertical distances, slope distances, inclinations, coordinates, bearings or azimuths, and multiple point traverse survey data. The instrument may also be coupled to a global positioning system (GPS) receiver to pinpoint the location from which measurements are being taken.
A new modularized surveying instrument system has been developed which has all of the features of the Criterion instrument and overcomes some of the operational limitations of the Criterion instrument. This modular system uses a fluxgate compass sensor in a separable compass module. The modular fluxgate compass in a separate module can be held level while the other modules are moved as necessary to take requisite measurements.
The fluxgate compass sensor must periodically be "zeroed" or corrected for variations in the inherent physical characteristics of the sensor. This must be done periodically as the internal characteristics of the sensor change over a period of time and exposure to stray magnetic fields and other interference effects interfere with true readings. The corrections are typically manually implemented calibration steps utilizing a known reference heading.
The fluxgate compass is basically a toroidal electromagnet core driven by a drive winding coil. A pair of secondary windings coils are positioned at 90 degrees apart around the toroidal core. The basic principle utilized in the fluxgate compass is to compare the drive coil current needed to saturate the core in one direction as opposed to the opposite direction. The difference between the saturation currents is due to the external field (the earth's magnetic field). The excitation is provided by a drive coil which periodically saturates the core. Variations in the core flux due to external magnetic field effects are then detected via the induced voltage in the secondary sensing coils. Since the secondary coils are spaced apart 90 degrees, one coil output is the "X" axis component output and the other coil output is the "Y" axis component output. These outputs, when appropriately demodulated, are proportional to x and y components of the magnetic field sensed.
A commercially available fluxgate sensor is available through KVH Industries, of Middletown, R.I. Their technique is to take these secondary signals and convert them to a time value through an integrator approach and then determine the x and y components of the sensed magnetic field signal strength from an integrating converter. This approach is complex and requires a great deal of circuitry to accomplish. A simpler approach is needed to determine the direction and magnitude of an external magnetic field utilizing a fluxgate sensor.
In order to conventionally zero calibrate the KVH fluxgate compass instrument the instrument may be placed at a known location in a known orientation and then its accuracy corrected to this known location. The instrument must then be periodically re-calibrated in a similar manner to correct for zero drift of the electronics due to component aging and other long term effects. This process is time consuming and somewhat complex. Accordingly there is a need for an instrument which automatically compensates itself for sensor circuit drift and other long term effects.